The present invention is directed toward a method and apparatus for controlling an apparatus for cell expansion. The cell expansion apparatus or bioreactor system comprises a closed loop cell expansion region where selected cells grow and multiply and a nutrient supply region that provides oxygen and nutrients to sustain the growing cells. The two regions are separated by a membrane. The present apparatus and method controls the bioreactor system by determining volume and rate of cell growth in the cell expansion region by monitoring the rate of nutrient use and the rate of change of the rate of nutrient use in the nutrient supply region.

Stem cells can be expanded from a few donor cells in a cell expansion apparatus. The resulting multiplied cells can be used to repair or replace damaged or defective tissues. Stem cells have broad clinical applications for a wide range of diseases. Recent advances in the area of regenerative medicine have demonstrated that stem cells have unique properties such as high proliferation rates and self-renewal capacity, ability to maintain an unspecialized cellular state, and the ability to differentiate into specialized cells under particular conditions.

As an important component of regenerative medicine, bioreactor systems play an important role in providing optimized environments for cell expansion. The bioreactor provides efficient nutrient supply to the cells and removal of metabolites, as well as furnishing a physiochemical environment conducive to cell growth. In particular, foreign cells, such as airborne pathogens, must be excluded from the cell-growth areas of the bioreactor. At the same time it is important to be able to determine how much cellular growth has taken place. Apparatus and methods for hermetically sampling expanding cellular material from cell-growth areas of a bioreactor, without environmental contamination, such as the apparatus and method of US Application No. 12/021,013, "Disposable Tubing Set for Use with a Cell Expansion Apparatus and Method for Sterile Sampling." Sensors in tubing of a bioreactor have been used to sense lactate levels as a predictor of the number of cells within the bioreactor, as disclosed in US Patent Application 12/536,707, "A Predictor of When to Harvest Cells Grown in a Bioreactor".

The present apparatus and method controls a bioreactor system by determining volume and rate of cell growth in the cell expansion region by monitoring the rate of nutrient use and the rate of change of the rate of nutrient use in the nutrient supply region. Rate of lactate creation and rate of change of rate of lactate creation may be used instead of or in conjunction with nutrient change. An in-line biosensor continuously senses conditions in selected lines, such as a waste line, to determine outflow conditions. In-flow conditions may be determined from pre-determined fluid concentrations and operation of pumps and valves. Preferably, out flow conditions are held relatively constant with regard to nutrient or lactate levels by feedback from a sensor in an outflow path through a computer controller, which adjusts the operation of pumps controlling in-flow. The pump rate or rates will have a changing speed and acceleration proportional to expected changes in nutrient level or lactate level, in the absence of added fluid. Thus, the speed and acceleration of the pump or pumps can be used as a surrogate for changes in the cell population in the bioreactor system. The number of cells in the bioreactor system can be derived from the speed of the pumps. The time needed to achieve growth to a selected number of cells may be calculated from the acceleration of the pumps.

The bioreactor system comprises a disposable apparatus for cell expansion, having at least one bioreactor. The bioreactor has a cellular growth area and a nutrient supply area, the cellular growth area being separated from the supply area by a membrane. The membrane inhibits migration of cells from the cellular growth area to the supply area and permits migration of certain chemical compounds from the cellular growth area to the supply area and of certain other chemical compounds from the supply area to the cellular growth area. At least one oxygenator is in fluid communication with the supply area, and a plurality of bags is in fluid communication with the cellular growth area, the bags providing fluids to the cellular growth area. A fluid recirculation path is in fluid communication with the cellular growth area.

Another aspect of the invention comprises a method of expanding cellular matter, the method comprising providing at least one bioreactor, the bioreactor having a cellular growth area and a supply area, and the cellular growth area being separated from the supply area by a membrane, the membrane being adapted to inhibit migration of cells from said cellular growth area to said supply area and to permit migration of certain chemical compounds from said cellular growth area to said supply area and of certain other chemical compounds from said supply area to said cellular growth area. The method further comprises conducting a fluid containing cellular matter into the cellular growth area; providing oxygenated fluid to said supply area to maintain conditions conducive for cell growth in said cellular growth area; monitoring inflow and outflow conditions; determining rates of change (first derivative) for selected conditions and calculating cell numbers therefrom; determining a rate of change of a rate of change (second derivative) for the selected conditions and calculating a predicted process time; and terminating the growth process when adequate cell numbers have been produced.

The method for controlling a cell expansion apparatus may comprise conducting a fluid containing cellular matter into a cellular growth area; providing fluid to said cellular growth area to maintain conditions conducive for cell growth in said cellular growth area; monitoring inflow fluid conditions to the cellular growth area; monitoring fluid conditions in said cellular growth area; determining rates of change for at least one selected condition and calculating cell numbers therefrom; determining a rate of change of a rate of change for the at least one selected condition and calculating a predicted process time; and terminating the growth process when adequate cell numbers have been produced.

The method may also comprise monitoring a pump rate for providing fluid to the cellular growth area to measure a glucose level or a lactate level. The glucose level or the lactate level may be measured in a waste line. In another aspect of the invention, the glucose level or the lactate level may be maintained at a substantially constant level by controlling the rate fluid is provided to the cellular growth area.

In a further aspect of the invention the rate fluid is provided to the cellular growth area may be controlled by controlling the speed of at least one pump. The rate of change of the rate of change may comprise determining the acceleration of the at least one pump.

These and other features and advantages of the present invention will be apparent from following detailed description, taken with reference to the attached drawings and claims.

• FIG. 1 is a schematic description of a cell expansion apparatus.
• FIG. 2 is a flow chart of a program for controlling the cell expansion apparatus.
• FIG. 3 is a graph representing lactate levels.
• FIG. 4 is a graph representing glucose levels.
• FIG. 5 is a graph of output lactate and glucose levels during a procedure with increasing levels of input media addition.
• FIG. 6 is a graph of input media addition in connection with the procedure of FIG. 5.
• FIG. 7 is a flow chart of a second program for controlling the cell expansion apparatus.
• FIG. 8 a chart representing lactate and glucose levels for a procedure.
• FIG. 9 is a chart of lactate generation and glucose consumption for the procedure of FIG. 8.
• FIG. 10 is a chart of intracorporeal fluid (IC) added to the cell expansion apparatus in the procedure of FIG. 8 and FIG. 9.

In the following description of the invention and in the accompanying drawings, like numerals refer to like parts. A bioreactor system 10 comprises a cell expansion module 12 coupled to an intracorporeal (IC) media bag 22 for replenishing fluids and an oxygenator 14 coupled to an extracorporeal (EC) media bag 16 for providing oxygen and removing waste.

The cell expansion module 12, or bioreactor, may be comprised of semi-permeable hollow fibers or flat sheet membranes enclosed in a housing. If hollow fibers are used, the fibers may be made of a biocompatible polymeric material such as Polyamix, which is a blend of polyamide, polyarylethersulfone and polyvinylpyrrolidone. Depending upon the type of cells to be expanded in the bioreactor, the fibers may or may not be treated with a substance to enhance cell growth and/or adherence to the membrane. The fibers may be held in place within the housing with polyethylene potting. The bioreactor housing has at least four openings into the interior of the housing. Two open into the intra-capillary or IC space, fluidly connecting to the interior of the hollow fibers, and two open into the extracapillary or EC space, fluidly connecting to the space surrounding the hollow fibers.

Cells may be grown in the IC space. The IC space with its minimum volume reduces the quantity of expensive media and expensive cytokines/growth factors required. The semi-permeable membrane allows transfer of metabolic components, waste and gases between the EC and IC compartments. The molecular transfer characteristics of the hollow fibers are chosen to minimize loss of expensive reagents from the IC side, while allowing metabolic waste products to diffuse through the membrane into the EC side to be removed. The EC space carries nutrients to the cells in the IC space, removes waste byproducts and maintains gas balance. The bioreactor may be attached to the rest of the disposable set with connectors made of polyurethane (Tygothane C-210-A).

The oxygenator 14 used may be any commercially available oxygenator. One possible oxygenator is manufactured by Gambro GmbH, Hechingen, Germany. The oxygenator may have a fiber count of 1820, an internal fiber diameter of 280 µm, an outer fiber diameter of 386 µm and an intercapillary fluid volume of 16 mL. The hollow fibers of the oxygenator are enclosed in a housing having four port openings. Inlet 20 and outlet 46 ports are fluidly connected to the interior (intercapillary or IC space) of the hollow fibers. Another set of inlet 48 and outlet 50 ports are fluidly connected to the space surrounding the hollow fibers (extracapillary or EC space).

Through the IC inlet port 20 of the oxygenator, an EC inlet line 18 is connected to deliver either fresh media from the EC media bag 16 or recirculated EC media to the oxygenator 14. Connected to the IC outlet port 46, the EC line 18 delivers oxygenated EC media to the EC inlet port 42 on the bioreactor 12. Connected to the EC inlet port 48 is a line 52 to a source of gas 54 (gas line). The EC outlet port 50 is open to the atmosphere, with a 0.22 µ in-line filter 56 to prevent microbes from entering and contaminating the closed system.

An EC media bag 16, which contains the media, which will flow through the EC side of the bioreactor, may be connected via a portion of flexible tubing (the EC inlet line) 18 to the IC inlet port 20 of the oxygenator 14. The EC inlet line 18 brings fresh EC media to the oxygenator 14 to be oxygenated. The EC inlet line 18 may be made of polyvinyl chloride with fluorinated ethylene propylene (PVC/FEP (sold as Tygon SE-200)).

An IC media bag 22 that contains the media that will flow through the IC side of the bioreactor may be connected via a portion of flexible tubing (the IC inlet line) 24 to the IC inlet port 26 of the bioreactor 12. The IC inlet line 24 brings fresh IC media to the IC side of the bioreactor. The IC inlet line 24 may also be made of PVC/FEP.

A vent bag 28 may be connected to the disposable set via flexible tubing 27 to collect any air initially in the system, before the system is filled with media and cells.

A cell input bag 30 contains the cells to be added to the bioreactor 12. The cell input bag 30 is connected to the IC inlet line 24, which delivers cells into the lumen of the hollow fibers via cell input line 29.

When the cells are ready to be harvested, they are flushed out of the IC outlet port 34 of bioreactor 12 through cell harvest line 31 and into a cell harvest bag 32.

The disposable tubing set also may include a length of tubing which acts as an IC circulation loop 36. The IC media flows out of the bioreactor 12 from the IC outlet port 34 through tubing loop 36 and back into the bioreactor through the IC inlet port 26. This loop 36 is used to recirculate the IC media though the hollow fibers. It may also be used to flush the cells out of the hollow fibers and reseed/redistribute them throughout the hollow fibers for further growth.

The IC recirculation loop 36 may contain a sample tube 38, for example, an additional length of tubing. This additional tubing enables small pieces of the tubing to be sterilely removed from the disposable set and the media inside tested for markers of cellular metabolism such as pH, glucose, lactate, electrolytes, oxygen and carbon dioxide content. The sample tubing 38 may be made of SANIPURE™ tubing (SEBS). The sample tube 38 may be solvent bonded into the IC loop 36 using cyclohexanone.

An EC recirculation loop 40 allows the media on the EC side of the bioreactor to be recirculated. The EC recirculation loop 40 allows EC media to flow out of the bioreactor from the EC outlet port 42 back into the bioreactor through the EC inlet port 44. This loop may be used to recirculate the EC media that surrounds the hollow fibers.

IC and EC media containing metabolic breakdown products from cell growth are removed from the system via tubing 58 into a waste bag 60.

As shown in FIG. 1, the tubing set may engage three or more pump loops that correspond to the location of peristaltic pumps on the cell expansion apparatus. In an embodiment, the tubing set may have five pump loops, corresponding to pumps P1-P5 on the apparatus. The pump loops may be made of polyurethane (PU (available as Tygothane C-210A)).

A cassette for organizing the tubing lines and which may also contain tubing loops for the peristaltic pumps may also be included as part of the disposable. Additional tubing lines (see 62) can be added as needed to enable specific applications such as reseeding/redistributing cells in the bioreactor. In order to control the passage of fluid through the disposable 10, manually operated clamps 64, 66 may be provided. In addition, microprocessor-controlled pinch valves 68, 70, 72, 74, 76, 80, 82 may be coupled to selected tubes of the disposable. Microprocessor-controlled pinch valves are available on blood processing devices such as the TRIMA^® apheresis machine, available commercially from the assignee of this invention. A TRIMA^® apheresis machine may be modified to accept the disposable 10 by removing a centrifuge ordinarily mounted within the TRIMA^® apheresis machine and placing the bioreactor and oxygenator within the machine as an incubator 81. Temperature sensors 86, 88, 90 and pressure sensors 92, 94, and 96 can be connected to selected tubes of the disposable 10 and placed in electrical communication with a microprocessor. It is to be understood that pumps, temperature sensors, pressure sensors and pinch valves are preferably connected to the disposable set only temporarily by contact. Manual clamps, on the other hand, are usually mounted on their respective tubes and may be delivered with the disposable.

With the disposable apparatus 10 mounted in the incubator 81, extracorporeal media is flowed throughout the apparatus 10, including all connecting tubes, first and second drip chambers 98, 100, the oxygenator 14 and the bioreactor 12. Pumps P1, P2, P3 and P4, controlled by the incubator 18 may be selectively activated to force fluid into sections of the disposable apparatus to prime the apparatus. After priming, intercellular media and cells, for example mesenchymal stem cells may be added from bags 22, 30 through the first drip chamber 98 and conducted into a cell expansion area of the bioreactor 14 and related tubing including recirculation path 36 and sample means 38. The hermetically sealed condition of the apparatus 10 is maintained by providing a vent bag 28 coupled to the first drip chamber 98 to accommodate variations in flow from the EC media bag 16, the IC media bag 22 and the cell input bag 30. Driven by pump P2, extra corporeal fluid passes through the oxygenator 14 where the fluid is infused with gas into a supply area of the bioreactor 12. The supply area is separated from the cell expansion area by a membrane that allows oxygen and other desirable chemical components to pass into the cell expansion area and allows waste products of the cell expansion process to pass by osmosis out of the cell expansion area while preventing cellular matter from crossing the membrane. The status of the fluid flowing through the supply area of the bioreactor is monitored by temperature sensor 88 and pressure sensor 94 as well as temperature sensor 90, which monitors the temperature of the fluid entering the oxygenator 14. An appropriate gas, such as oxygen, or a gas mixture is conducted through the oxygenator 14 at a pressure monitored by sensor 96. The gas is preferably medical grade and is also isolated from ambient air by 0.22 micron filters 56, 57. The characteristics of the extracorporeal fluid can also be checked by withdrawing fluid samples through sample ports S1 and S2 on the inflow and outflow lines of the bioreactor. The sample ports have internal filters that allow fluid to be extracted by cellular sized particles from passing into or out of the apparatus 10.

The pumps are preferably peristaltic pumps. In addition to the manual clamps and automatically controlled valves, the pumps also act as valves, preventing flow of fluid past the pump when the pump is not actively driven. Therefore, when pump P1 is not in operation and valves 76, 78 are closed, a recirculation loop is formed through the bioreactor 12 and pump P4. Conditions in this recirculation loop, where cells are growing, are monitored with temperature sensor 86 and pressure sensor 92 and by taking fluid samples through a sample port S3. The fluid extracted through the sample port S3, as explained above, does not contain cells or particles of cellular size. It is important to be able to monitor the progress of cellular growth over time without compromising the hermetically sealed conditions of the apparatus 10. Once the desired cell concentrations have been obtained, the contents of the bioreactor 12 can be harvested into the cell harvest bag 32.

During an expansion cycle, some amount of fluid is typically removed from the fluid circulation paths (the IC and/or EC circuit) at various times throughout the cell expansion cycle and analyzed for the amount of metabolites and other by-products of cell growth in the fluids. The fluid removed from the fluid flow circuits may be run through any commercially available blood gas analyzer (the blood gas analyzer used in this instance was a Siemens 800 series) to measure the amounts of metabolites contained in the fluid. Using a blood gas analyzer, the concentration of lactate (or glucose) is measured in mM/L. Other methods of measurement such as direct chemistry may also be used.

Metabolites may also be measured using a biosensor. Any commercially available biosensor may be used. If the biosensor is sterile, or is made of a material which may be sterilized with ethylene oxide or gamma irradiation, it may be fluidly connected directly into the fluid lines (in-line). If the biosensor is not able to be sterilized, it may be indirectly connected into the fluid lines via a sterile barrier filter. Glucose and lactate molecules are small enough that they diffuse equally across the membrane, and are in equilibrium. Therefore, accurate measurements can be taken by any means on either the IC or EC side, or in waste line 58. Fluid may be removed from the IC loop 202 through sampling port 216 or sample coil 218 and/or from the EC loop 204 through sample port 230.

Aerobically growing cells consume glucose and oxygen and produce lactate. The more cells that are present in a cell growth chamber, the more glucose and oxygen are consumed and lactate generated. When cells are at a high density, particularly adherent cells, cell expansion slows due to increased cell-cell interaction between colonies. Cell clumping or aggregation also occurs at high cell density. It is currently not routine practice to look directly inside a cell growth chamber to see if cells are growing into each other without destroying the sterility of the system. Therefore, it would be advantageous if metabolic products of cell growth such as lactate, or products consumed during cell growth such as glucose and oxygen could be used as an indirect measurement to determine if cells were reaching confluence and should be harvested.

An algorithm to determine the number of cell doublings, which, in turn, determines the best time to reseed or harvest the cells before cell growth slows has been disclosed in US Application 12/536,707. The number of doublings can be determined using lactate mass generated and the number of cells initially loaded into the cell expansion system.

The present apparatus and method controls a bioreactor system by determining volume and rate of cell growth in the cell expansion region by monitoring the rate of nutrient use, such as glucose, and the rate of change of the rate of nutrient use in the nutrient supply region. Rate of lactate creation and rate of change of rate of lactate creation may be used instead of or in conjunction with nutrient change. An in-line biosensor 102 may continuously sense conditions in selected lines, such as the waste line 58, to determine outflow conditions. A suitable sensor for detecting glucose or lactate concentration is available from Jobst Technologies GmbH, Freiburg, Germany. Either a glucose sensor or a lactate sensor or both could be used. In-flow conditions for glucose concentrations and volumes may be determined from pre-determined fluid concentrations and operation of pump P5 and valve 72.

As shown in FIG. 1, a digital computer 104 is receives data from the sensor 102 and controls valves and pumps thereby regulating the addition of fluids into and out of the bioreactor. The digital computer 104 may include memory, at least one processor, and a user interface for receiving instructions from a user via a user input device (mouse, keyboard, keypad, touch screen, optical sensor or verbal command). In addition, the digital computer 104 comprises a CES controller interface for relaying information to and from elements such as sensors (pressure, temperature, and biosensor) and for instructing various mechanical systems such as the pumps and valves. The digital computer 104 may be in communication with additional sensors for monitoring other aspects of the apparatus, such as whether one or more fluid bags are low and/or empty. Programming utilized by the digital computer 104 may comprise, by way of example and not limitation, software or firmware.

FIG. 2 represents an algorithm 110 for controlling the bioreactor system 10, and, in particular, for predicting a time for process completion from the rate of change of the rate of change of a selected parameter (the second derivative or acceleration), such as glucose level or lactate level or pump speeds.

Change of glucose in the system may be determined by measuring 112 the volume of solution delivered to the system from the IC media bag 22 or the EC media bag 16. The volume of solution delivered to the system may be determined from the revolutions of a peristaltic pump coupled to an inflow line containing glucose solution. The contents of the media bags 16, 22 should have a known concentration of glucose solution. Alternatively, one or more sensors could be provided to detect the glucose concentration at the media bags. Glucose removed from the system may be measured 114 by sensing both glucose concentration and fluid volume of fluids being removed to the waste bag 60 past sensor 102. Multiple measurements correlated with a timer, which may be integrated into the controller or digital computer 104, allow both the rate of change of glucose concentration (first derivative or "velocity" of change) 118 and the rate of change of the velocity of change (second derivative of "acceleration" of change) 120 to be calculated.

Lactate levels may also be measured 116, either in conjunction with the measurement of glucose levels, or as an alternative. Since lactate is a product of the cells in the system, the initial condition is that there should be essentially no lactate in the system. Like glucose, lactate removed from the system may be determined by sensing both lactate concentration and fluid volume of fluids being removed to the waste bag 60 past sensor 102. Multiple measurements correlated with the timer allow both the rate of change of lactate concentration (first derivative or "velocity" of change) 122 and the rate of change of the velocity of change (second derivative of "acceleration" of change) 124 to be calculated.

In order to encourage optimum growth, the bioreactor system 10 tries to maintain an initial or optimum growth condition. This may be approximated by periodically adding nutrient fluid to the system and by removing an equivalent amount of fluid from the system, less losses from leakage or evaporation. This process is illustrated in FIG. 3 and FIG. 4. FIG. 3 illustrates two curves 134, 140. A smooth, continuously rising curve 134 represents increasing lactate level with increasing time at the waste line output sensor 102 or elsewhere within the system, if there were no additional fluid added or removed from the system. In the absence of changing conditions such as adding new fluid, the concentration of lactate would be expected to rise from an initial condition 136 (low or near zero lactate) as a function of the increased number of cells and accumulation of lactate in the system. Of course, if glucose and other nutrients are not added to the system and if wastes are not removed, lactate accumulation will slow or stop. For purposes of this illustration, however, the bioreactor system 10 does add nutrients and remove waste. The actual concentration of lactate in the system 10 is illustrated by dotted line 138. Both dL/dt and d²L/dt² can be calculated from the measurements described above and from the changes in the pump action adding fluid to the system. Fluid addition to the system is represented by the curve 140. Rate of change of the lactate level (dL/dt) is proportional to the number of cells in the system 10. The rate of change of the rate of change (acceleration or d²L/dt²) of the lactate level is proportional to the duration of the process until a desired number of cells will have been grown. This can be used to predict an end time for the process on an on-going, updated basis, thereby reducing the need for continuous monitoring of the system by a human operator. As can be seen in FIG. 3, the fluid addition line 140, which is directly related to the speed of the pump P5, is directly proportional to the expected lactate level 134, when the lactate level is held relatively constant, as shown by line 138. A similar process may be employed with respect to the glucose level, as illustrated in FIG. 4. In this case, a smooth, continuously falling curve 156 represents decreasing glucose level with increasing time. In the absence of changing conditions, the concentration of glucose will fall from an initial condition 158 as a function of the increased number of cells and consumption of glucose in the system. Of course, if glucose and other nutrients are not added to the system, the glucose will eventually be completely consumed. As before, the bioreactor system 10 does add nutrients and remove waste. The actual concentration of glucose in the system 10 is illustrated by dashed line 160. The rate of media addition or the pump speed is represented by rising line 162, which is congruent with line 140 of FIG. 3. The rate of media addition 162 (which is directly proportional to the pump speed), when the glucose level is held constant, is inversely proportional to the expected glucose level if no media were added. Thus, both dG/dt and d²G/dt² can be calculated from the measurements described above and from the action of the pump or pumps adding fluid to the system. Rate of change of the glucose level (dG/dt) is inversely proportional to the number of cells in the system 10. The rate of change of the rate of change (acceleration or d²G/dt²) of the glucose level is inversely proportional to the duration of the process until a desired number of cells will have been grown. This can be also be used to predict an end time for the process on an on-going, updated basis, thereby reducing the need for continuous monitoring of the system by a human operator.

An implementation of this process is illustrated in Figures 5 and 6 wherein amounts of IC fluid and EC fluid were added continuously over a period of eighteen days to a cell growth apparatus. The glucose level was adjusted towards 80 mg/dL. The lactate level was generally held below 9 mmol/L. The levels of lactate and glucose and the lactate generation rate were measured over the 17.62-day growth process, as shown in the following table. Table 1. Lactate and Glucose Levels Time (Days) Lactate (mmol/L) Glucose (mg/dL) Lactate Generation (mmol/day) 0.00 1.53 91 0 2.70 2.36 85 0.14 3.66 2.8 82 0.29 3.95 2.93 81 0.32 3.96 1.93 91 4.72 2.77 81 0.61 4.95 3.36 76 1.45 5.62 4.48 68 1.08 5.94 5.11 62 2.21 6.61 5.33 60 1.52 6.81 5.56 57 2.04 6.84 2.19 88 8.13 7.82 43 3.58 9.64 8.43 25 5.44 9.70 8.38 26 5.02 9.85 7.56 30 2.05 10.02 7.41 34 4.23 10.77 7.66 37 4.90 10.90 7.87 39 5.77 10.92 2.88 77 10.99 3.8 70 8.48 11.61 6.95 46 5.71 11.79 5.44 54 6.94 11.95 5.12 53 7.83 12.65 5.04 67 8.31 12.97 4.8 62 7.44 13.64 5.5 62 8.98 13.94 5.19 61 8.37 14.01 2.1 91 14.73 4.12 63 7.46 15.13 4.8 64 12.67 15.81 4.85 61 13.35 16.65 5.03 58 13.92 16.96 5.1 63 17.33 17.62 5.03 68 17.15

As shown in FIG. 6, the rate of EC fluid addition in mL/minute was incrementally increased over the period. Although this graph shows discrete changes in fluid addition rate, it is clear that mathematical methods could be used to fit a curve to the discrete data. Moreover, with the control feedback described herein, a smoother addition rate curve would be developed, and both the lactate and glucose levels shown in FIG. 5 could be held within closer tolerances.

The continuous addition of new solution to the system is also shown in the schematic 150 of FIG. 7. The level of lactate at the waste bag 60 would preferably be substantially constant. Deviations would be corrected 166 by the controller or digital computer 104 increasing or decreasing the rate of a pump P5 coupled to the IC media bag 22 or the EC media bag 16 through valves 72, 70, respectively. Once again, the amount of fluid added during the procedure will rise exponentially, preferably continuously. The speed at which fluid is added and the acceleration of fluid addition can be known from the speed and acceleration of the pump or pumps. Either lactate level or glucose level or both could be measured 114, 116 and corrections 166 made in the pump speed, such as acceleration, to maintain the lactate and glucose levels at relatively constant levels. After the corrections, a smoothed curve representing the speed of the pump or pumps may be calculated 168 and a curve representing the acceleration of the speed of the pump or pumps can be calculated 170 and used to further calculate 128 the number of cells in the bioreactor (from the speed of the pump or pumps) and to predict 130 the time at which the desired number of cells will be grown (from the acceleration of the pump or pumps).

In one implementation with continuous IC fluid addition, an initial aliquot of about 20 million mesenchymal stem cells was loaded in a cell expansion system. After a five and a half day growth period, about 184 million cells were harvested. Table 1 records lactate and glucose levels and use over the growth period. Table 2. Continuous IC Fluid Addition Time (days) Lactate (mmol/L) Glucose (mg/dL) Lactate Generation Glucose Generation 0.00 1.69 92 0.000 0.000 0.65 2.22 89 0.424 0.271 0.81 2.12 88 0.331 0.90 1.89 92 1.63 2.56 86 0.507 0.360 1.93 2.79 86 0.492 0.150 2.64 3.67 78 0.795 0.493 2.96 4.15 76 1.025 0.392 4.02 5.04 64 1.191 0.791 5.16 4.64 74 3.368 1.536 5.64 3.44 76 4.077 2.631

As seen in FIG. 8, the lactose level was held relatively constant with values throughout the growth period between 1.5 mmol/L and 5 mmol/L. At the same time, the glucose level was held relatively constant, falling between about 92 mg/dL and 62 mg/dL. The rates of lactate generation and glucose consumption, illustrated in FIG. 9, showed a similar, increasing trend, to which a curve could be fitted and rate of generation/consumption and acceleration of generation/consumption could be calculated. As described above, this information could be used to estimate the number of cells in the system (from the rate of consumption) and the time needed to grow a desired number of cells (from the acceleration). FIG. 10 shows that the rate of fluid addition, which is known from the pump rates, shows a similar, geometrically increasing shape. The magnitude of the curve at a selected time is proportional to the speed to the pumps, and the slope of the curve is proportional to the acceleration of the pumps. Consequently, the number of cells in the system at a given time can be derived from the speed of the pumps. The time needed for the growth of a selected number of cells can be derived from the acceleration of the pumps. The system would be able to display these values to an operator either continuously or at discrete intervals. This allows improved monitoring of the system and improved prediction of the time when the cells should be harvested.

The foregoing description of the present invention has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and skill and knowledge of the relevant art, are within the scope of the present invention as defined in the claims. The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such embodiments, or other embodiments and with various modifications required by the particular application(s) or use(s) of the present invention.